The capacity enlargement of a backbone optical communication network has become a problem in order to deal with an explosive expansion of the information-communication traffic forecast in the future. A variety of approaches have been taken to the problem, and as one of those approaches, the research and development to enlarge the capacity and improve the flexibility of an optical node has been performed.
In order to enlarge the capacity of an optical node, there is a method of enlarging the communication capacity per optical transmission line, that is, optical fiber, and a method of laying an optical fiber itself additionally. The method of enlarging the communication capacity per optical fiber has been realized by a time division multiplexing (TDM) method or a wavelength division multiplexing (WDM) method. The method of laying an optical fiber itself additionally is called a space division multiplexing (SDM) technique.
The communication between user points is performed by connecting the user points by an optical path which is identified by a wavelength or an optical fiber. The optical path is composed of a single optical fiber or a combination of a plurality of optical fibers. When a plurality of optical fibers are combined, the combination is realized by an optical node using a reconfigurable optical add drop multiplexer (ROADM) or the like. That is to say, changing the combination of a plurality of optical fibers dynamically makes it possible to realize the communication between various points. Specifically, such communication is realized by switching connection between an input optical fiber of an ROADM and one of various output optical fibers by using an optical switch which is a component of the ROADM. This is generally called an optical path switching in an ROADM.
In order to enlarge the communication capacity of a network as referred to above, it is believed that there are two methods, that is, a method of enlarging the communication capacity of an optical path itself and a method of increasing the number of optical paths. It is possible to apply the above-mentioned TDM technique in order to enlarge the communication capacity of an optical path itself. In order to increase the number of optical paths, it is possible to apply the above-mentioned WDM technique or SDM technique.
The speeding up of an electrical circuit is necessary for enlarging the communication capacity by the TDM technique. However, the speeding up of an electrical circuit has almost reached a technical limit in recent years, and it becomes difficult to improve it dramatically.
It is necessary for enlarging the communication capacity by the WDM technique to widen the optical communication bandwidth or to decrease the optical bandwidth to be allocated to each optical path. Regarding the optical communication bandwidth, it is not easy to widen the bandwidth because there are limits due to a loss of an optical fiber or an amplification bandwidth of an optical amplifier. Therefore, an approach has been adopted in recent years that dense wavelength multiplexing is performed by decreasing the optical bandwidth used in a single optical path.
On the other hand, it is necessary for enlarging the communication capacity by the SDM technique to lay an optical fiber additionally. The method using the SDM technique is a method for enlarging with most room for enlargement because there is no significant technical issue in the SDM technique unlike the TDM technique or the WDM technique. However, if the fault tolerance is improved by preparing different optical fibers for an active system and a standby system, optical fibers are required up to twice the number of those required actually.
If the communication capacity of a network is enlarged by the SDM technique, it is indispensable to increase the number of optical transceivers and the number of input-output ports of an optical node because of the increase in the number of optical fibers. In order to increase the number of input-output ports of an optical node, it is necessary to increase the number of input-output ports of an optical switch which is a core element of the optical node. For this purpose, it is necessary to combine a plurality of optical switching elements. Therefore, the number of required optical switching elements increases as the number of input-output ports increases. Furthermore, if optical signals are wavelength multiplexed for each input fiber (for each route) toward an optical node, it becomes necessary to perform a switching operation for each wavelength at the optical node. For this reason, it is performed to wavelength demultiplex inputted optical signals and to switch demultiplexed optical signal depending on the wavelength and the route in the optical switch at an optical node. Therefore, the maximum number of input-output ports and the number of optical switching elements required for an optical switch are determined by the number of available routes of the optical node and the wavelength multiplicity number of optical signals. For example, if an ROADM deals with four routes and 80 wavelengths multiplexing, and the add/drop rate is assumed to be 100%, the ROADM requires about two hundred thousand (≈(4×80)×(4×80)×2) optical switching elements. Since it is not practical to combine no less than about one hundred thousand optical switching elements in terms of physical size, integration utilizing a planar lightwave circuit (PLC) technique is carried out.
As mentioned above, a technique to improve the flexibility of an optical node is known as another approach for enlarging the capacity of the backbone optical communication network. The optical path at the optical node is switched depending on the wavelength and the route. In order to improve the flexibility of the optical node, it is necessary that an optical path having an arbitrary input wavelength and an arbitrary route can be connected to an arbitrary output with non-blocking. In recent years, the research and development of a CDC-ROADM technique has been performed as a technique to realize the above, and an example of the technique is described in Non Patent Literature 1. CDC stands for “Colorless”, “Directionless”, and “Contentionless”. “Colorless” means a function of making it possible to input an optical signal with an arbitrary wavelength into an arbitrary input port of an ROADM, and to output an optical signal with an arbitrary wavelength from an arbitrary output port. “Directionless” means a function of making it possible to guide an input optical signal to an arbitrary route. “Contentionless” means a function of avoiding collision with optical signals having the same wavelength in an ROADM. That is to say, “CDC” function means a function of making it possible to connect an optical signal from an arbitrary input port to an arbitrary output port without conflict over an input port and an input wavelength, without re-configuration, and with non-blocking.
As mentioned above, in order to build a large-capacity and flexible optical communication network, it has made progress in recent years to enable an optical node to have multi-routes and CDC functions. It has been developed to increase the number of input-output ports of an integrated optical switch supporting CDC functions, that is, to enlarge its scale.
Non Patent Literature 2 describes an example of the optical switch supporting CDC functions. The related optical switch described in Non Patent Literature 2 has a configuration obtained by combining, in a tree-shaped structure, waveguide optical switching elements, each of which has one input port and two output ports and is a split-and-select type optical switching (SS optical switching) element. That is to say, the SS optical switching element having one input port and two output ports (1×2) structure performs a switching operation of optical signals by splitting input light into two light beams and selecting one of the split light beams.
A multi-stage connection of SS optical switching elements makes it possible to realize a 1×N optical switch (where N is a natural number more than one: N≧2). Connecting M pieces of 1×N SS optical switches in parallel (where M is a natural number more than one: M≧2) makes it possible to realize an N×M SS optical switch. With respect to the number of optical switching elements, it results in integrating optical switching elements equal to or more than 100 to configure an 8×8 SS optical switch by connecting eight pieces of 1×8 switches in parallel, for example.